Lawrencium

Lawrencium, the last of the actinides, was discovered as a result of producing a few atoms of element 103. This discovery increased the actinide elements to fifteen, matching the number of lanthanides and filling out the f-block in the periodic table.
 Sikkeland, Larsh, and Latimer United States 1961 Lawrencium

Lawrencium, the last of the actinides, was discovered as a result of producing a few atoms of element 103. This discovery increased the actinide elements to fifteen, matching the number of lanthanides and filling out the f-block in the periodic table.